This invention relates to an improved power monitor for use in starting and stopping a digital electronic system which has data storage, and more particularly to a monitor for providing a low impedance path to ground for initializing the digital system at A.C. power turn-on and A.C. power turn-off.
Data processing systems require a number of carefully regulated D.C. voltages which must be within certain specified ranges in order for the electronics to operate properly. For example, the regulated 5 volt supply must be within .+-.5% for a reliable operation. In a typical digitally-based logic system, initialization following turn-on is enabled by a signal which presents a low impedance path to ground. This signal is held for several milliseconds following power turn-on to insure that the various regulated D.C. output voltages have reached their nominal values and that the logic circuits are started in defined states. In prior art devices of which applicant is aware, this function is provided by the closing of relay contacts which provide a low impedance path to ground.
In addition to a requirement for providing an initializing signal at turn-on, an initializing signal is in accordance with this invention also provided at power turn-off. The reason for this later requirement is that the regulated power supply will continue to provided output voltages following power turn-off; however, these outputs will tend to degradate from their nominal values and cause erroneous operation of the logic circuits.
The object of the present invention is to provide a relatively simple, inexpensive solid state circuit which provides a low impedance initializing signal when power is turned on and when power is turned off.
Briefly, the present invention contemplates the provision of a low impedance FET switch coupled between the digital electronic system and ground. This FET is coupled to the regulated D.C. power supply and conducts following A.C. power turn-on until cut-off bias is applied to its base. This bias is divided via a delay return from the regulated D.C. supply. At A.C. turn-off, the cut-off bias is removed from the base of the FET before the outputs of the regulated D.C. supply fall out of the range of acceptability.